This was written in Python 2.7.6 (way back when Codewars only supported Python 2.7.6), so try to be less rude when you're wrong. Or all the time, really. kthxbei

Also using zip adds an extra iteration over the lists.

wrong, now that python 3 use zip as a generator/iterator.

To count you need a list, here is generator expression.

I think it's perfectly fine, this is exactly why bool is a subclass of int. If the upcasting upsets you so much then you can just do sum(1 for ch1, ch2 in zip(a, b) if ch1 != ch2).

Don't look at me; I'm using CodeWars to learn Python. ;-)